Rodents and monkeys and apes, 0h my: comparative and experimental investigations of systemic skeletal robusticity in rodents and primates

1Institute of Human Origins, Arizona State University, 2School of Human Evolution and Social Change, Arizona State University, 3Department of Biology, University of California, Riverside, 4Department of Orthopaedic Surgery, University of Michigan, 5Department of Biomedical Engineering, State University of New York, Stony Brook, 6Department of Biology, California Polytechnic State University, San Luis Obispo

Increased cranial vault thickness in Homo erectus and some Homo heidelbergensis is an intractable paleoanthropological conundrum that has proved resistant to explanation for over 100 years. One hypothesized mechanism, systemic skeletal robusticity mediated by exercise-induced growth factors, has never been rigorously tested. We present data from 4 experiments involving mice or rats that directly tested the relationships among locomotor activity, circulating growth hormones, and systemic skeletal robusticity. In no group was circulating IGF-I, IGFBP-3, or their ratio significantly associated with measures of skeletal robusticity. Some measures of femoral and humeral robusticity were correlated with some measures of cranial vault thickness in some groups – offering limited, but intriguing support for the hypothesis. For example, in the rat study, total cortical thickness of the parietal bone was weakly correlated with femoral cortical : total area ratio (r=0.375, p=0.025), but total parietal thickness was not statistically associated with any measure of postcranial robusticity.

Systemic robusticity was also examined in modern humans, Trachypithecus cristatus, and Hylobates lar. No hormone information was available, but measures of cranial and postcranial robusticity were not significantly correlated in any species.

Our projects demonstrate that model organisms can prove useful in addressing classic questions of paleoanthropological interest. Comparative primate morphology alone could not have fully and rigorously tested the hypothesis. These studies are the first to integrate data from multiple animal models investigated in controlled experimental settings. Together, they suggest that activity-mediated hormonal control of systemic robusticity remains an intriguing, but incomplete, explanation for Pleisotcene cranial hyperostosis.

Funding for this project was provided by the NSF (grants to LC, TG), the Wenner-Gren Foundation (to LC), the NIH (to SJ, KJ), the Beinecke Family Foundation (to LC), and the Institute of Human Origins (to LC).